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Prediction of Dissolution Time of Gerromanganese in Hot Metal and Steel Bath

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Prediction of Dissolution Time of Gerromanganese in Hot Metal and Steel Bath PROCEEDINGS: CAMME-1996 ©NML JAMSHEDPUR; pp.81-98 Prediction of dissolution time of gerromanganese in hot metal and steel bath RASHMI KUMAR and SANJAY CHANDRA The Tata Iron & Steel Company Limited, Jamshedpur - 831 001, India ABSTRACT Mathematical models enable us to study metallurgical processes in depth. Significant improvements in plant operation cannot, in general, be brought about without quantification of the impact of the operating parameters on the end product. It is for this reason that mathematical models have become popular in the steel industry, particularly with the advent of low-cost, high speed personal computers. Another reason why modelling is necessary is because increasingly alterations in op- erating parameters have to be made to bring about changes in heat or mass transfer. The gut-feelings of the plant operator are no longer able to deliver the goods because such feelings are developed by re- peated exposure to quantifiable changes in the process brought about by alterations, deliberate or otherwise, in the operating parameters. Some of these basic parameters are not measured, for example heat transfer coefficient in case of a solid particle .dissolving in a liquid bath, and therefore it would be futile to expect gut-feelings to offer solutions as to how dissolution time would be effected by a change in the type of ferroalloy or by change in its size. The determination of the dissolution time of ferroalloys is important as it indicates to the pro- cess control engineer the minimum time necessary before the bath can be subjected to the next step of processing. Changes in the bath super- heat; the size and the initial temperature of the ferroalloy and the bath hydrodynamics influence the dissolution time. Determining this disso- lution time by experimentation is extremely difficult and would involve a series of experiments with different types of ferroalloys under vary- ing conditions. On the other hand, the dissolution time can be obtained using a mathematical model of the dissolution kinetics of ferroalloys which after proper validation can be used to assess the dissolution behaviour of the ferroalloy under a variety of conditions. At TATA STEEL blast furnace hot metal is used to cast ingot molds. The com- 81 RASHMI KUMAR and SANJAY CHANDRA position of the blast furnace hot metal desired for making ingot moulds demands the addition of 20 kg each of ferromanganese and ferro-sili- con in the 12 tonne transfer ladles into which the blast furnace metal is poured at the ingot mould foundry (IMF). Owing to the low tempera- ture of hot metal at IMF, casting commences immediately and a com- plete dissolution of the ferroalloy is often not possible. A mathematical model for calculating dissolution time for a solid particle in a liquid metal has been developed at R&D Division of Tata Steel. The model has been developed with the feasibility of predicting dissolution times for eases where the ferroalloy has a melting point lower as well as higher than the bath temperature. The model has been validated against data published in literature and applied to predict dissolution times of ferroalloys in steel and blast furnace hot metal. This model was used to study dissolution of ferroalloys in foundry and to evolve suitable corrective measures. On the basis of this study additions are being optimized at the IMF in' Tata Steel. INTRODUCTION Tata Steel has adopted a philosophy of steelmaking in which hot metal is desulphurised prior to its charging into the BOF vessel. Addi- tionally, a low managanese hot metal is intentionally produced to . minimise the detrimental impact of Mn on the phosphorus partition ratio during steelmaking. On the other hand,. for adequate life of ingot moulds, which are cast in the captive foundry of the steel plant using blast furnace hot metal, high silicon and high manganese-to-sulphur ratio in the hot metal are necessary. To be able to meet these conflict- ing demands, one of the blast furnaces is burdened with manganese ore and a slightly higher ore to coke ratio so as to produce hot metal suited for casting of ingot mould. Nevertheless, in order to exactly meet the desired cast iron composition at the foundry, trimming additions of ferromanganese and ferrosilicon are necessary. Addition of ferromanganese is also carried out in the BOF shop after the steel has been tapped from the LD vessel. The addition is done in the 130 tonne ladle into which the steel is transferred after heat making. In both the cases discussed above, it becomes necessary to have a quantitative picture of the time taken for the ferroalloys to dissolve in the bath as a function of the superheat of the bath, the purging inten- sity employed and the size of the ferroalloy itself. Such a quantifica- tion is not possible without taking recourse to an elaborate mathemati- 82 RASHMI KUMAR and SANJAY CHANDRA cal model, primarily on account of the fact that an external casing (or solidified shell) of the melt covers the ferroalloy as soon as it is im- mersed in'the bath. Thereafter, the heat transfer to the steel (or cast iron) covered additive is no longer the large temperature difference between the molten bath and the cold ferroalloy. The driving force for heat transfer is considerably reduced to a small difference in tempera- ture between the molten bath and the steel (or cast iron) casing around the additive. Thus to quantify the effect of various process parameters on the dissolution period of ferroalloys, a mathematical model incorporating the initial formation of a solid shell, its growth and its subsequent melting has been developed at the R&D Division at Tata Steel. This paper details the development of the model, its validation and the benefits that are likely to accrue from the model's recommendations. FERROALLOY DISSOLUTION MECHANISMS It is well known that when a cold particle is immersed into a liquid metal at elevated temperatures a solid shell or crust is immediately formed around it. This shell continues to grow so long as the rate at which it transfers heat away from it is higher than the rate at which it receives heat from the bath. The growth stops and the melting of the solid crust begins when the rate of heat transfer from the melt by convection becomes greater than the rate of heat conduction through the. crust. There are various ways in which lumps of ferroalloys dissolve in a liquid melt. Two broad categories, depending on whether the melting range of the ferroalloy lies below. or above the liquid metal bath tem- perature, have been identified by Guthrie and co-workers" 1. Class I Ferroalloy: A typical class I ferroalloy has been defined as one whose melting point lies below the bath temperature of the melt. Absorption of such ferroalloys in the bath occurs via melting. This class includes ferromanganese, silicomanganese, ferrochrome and ferrosilicon addi- tion to steel melts and tin, antimony, copper manganese, nickel and silicon additions to molten cast iron. During the assimilation of these ferroalloys into the bath, the most likely series of thermal events (four in number) are shown in an idealized form in Figure 1. 83 RASHMI KUMAR and SANJAY CHANDRA Route 2 Route 3 = 0 Route 4 Route 5 Fig. I : Five kinetic paths for alloy additions meltinct and/or dissolving in molten steel Fig. 2: Schematic representation of an addition with the solidified Metal shell and hot metal (various interfaces and the coordinate system used in the mathematical . model are also illustrated). 84 RASHMI KUMAR and SANJAY CHANDRA In route 1, as the cold addition (IA) is plunged into the metal bath, it gets. covered by a shell of solidified metal (1B). The ferroalloy be- gins to melt within the steel shell, even as the steel shell continues to grow (1C). Very often, the solid added becomes totally molten before the shell melts back and releases the melted portion to the steel bath. The rate of shell melt back is governed by the convective heat transfer from the bath and thus, depends on the bath hydrodynamics (i.e., stir- ring). Occasionally, alternative routes such as 2,3 or 4, may be followed depending on the specific thermal properties of the ferroalloy, size of the addition, and the composition and temperature of the bath. Condi- tions favouring route 2, in which no second metal shell is formed once the solid portion of the alloy gets exposed to the metal bath, include high superheat temperatures, larger lumps (-15 cm) and ferroalloys with low Thermal conductivity (-4 W/m/K). If the ferroalloy is not fully molten by the time the first metal shell has melted back (3C), the exposed remainder of the lump generally follows route 3 and becomes covered once again with a second, smaller steel shell (3D). This type of phenomena is associated with large lumps of the additive and high superheat of the melts. Route 4 depicts the case of ferroalloys having high exothermic heats of dissolution in liquid steel. The best known examples of this are the ferrosilicon alloys which, on being plunged into a steel bath (4A), exhibit an exothermic dissolution reaction triggered at the inner steel shell/solid ferrosilicon interface (4B). Route 4 is a possibility for both class I and II ferroalloys exhibiting exothermic dissolution behav- ior. Class II Ferroalloy: Class II ferroalloys are those whose melting points or melting ranges lie-above the bath temperature of liquid melt. Typical examples include ferrovanadium, ferrotungsten and ferromolybdenum additions to a steel melt and chromium, molybdenum, vanadium and niobium additions to cast iron melts. As is normal, a metal shell forms around such an addition following immersion (5B).
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